Pipe mold and method of making the same



Dec. 4, 1951 R. E. LUDWIG ETAL 2,577,423

' PIPE MOLD AND METHOD OF MAKING THEVSAME 'Filed May 51. 1949 Patented Dec. 4, 1951 PIPE MOLD AND METHOD or MAKING THE SAME Ralph E. Ludwig, Youngsville, and Herbert L. MacBride, Warren, Pa., assignors to National Forge & Ordnance Company, Irvine, Pa., a corporation of Delaware Application May 31, 1949, Serial No. 96,335

1 This invention relates generally to metal molds and in particular to' those used in the manufacture of cast iron pipes by the well known cen-' trifugal casting process.

The mold which consists of a cylindrical tube made from metal and usually provided with a bell mouth at one end is supported horizontally on a movable carriage for rotation on its major axis, and the pipe is formed by pouring molten pipe metal onto the inner surface of the rotating mold from a stationary delivery spout which extends through the mold while' simultaneously moving the mold in a longitudinal direction. Centrifugal forces arising from rotation of the mold distribute the molten metal to a substantially uniform thickness around the bore. When the pipemetal has cooled and solidified, the pipe thus formed is stripped from the mold and the latter readied for a repeat molding operation. Despite the well established practice of applying a coating compound to the inner surface of the mold prior to casting of the pipe, the temperature of the latter nevertheless rise to a comparatively high level at each pouring of the pipe metal. The outer wall of the mold, however, remains much cooler with the result that the inner wall is unable to expand uniformly to relieve stresses caused by the heating effect. That is to say, the innermost layers of the mold wall which would normally expand most during the casting are prevented from doing so by the relatively much cooler outer layers of the mold wall thus setting up a pattern of unrelieved stresses throughout the length and circumference of the mold which may exceed the elastic limit of the mold metal. Subsequently, after the pipe' has been cast and the latter withdrawn from the mold, the interior surface layers of the mold cool giving rise to contraction stresses but these are also unable to relieve themselves because of the cooler exterior layers of the mold.

It is believed that this cyclic heating and cooling of the mold accelerates fatigue failure in the interior layers of the mold metal for it has been found in practice that after the mold has been in use for some time, the interior mold'surface, which heretofore when new has been finished to a relatively high degree of smoothness, begins to exhibit a certain roughness and ultimately one or more slight fissures make their appearance. As use of the mold continues, the cyclic compression and tension stresses would appear to concentrate their effect at the largest of the fissures already made with the result that this particular fissure becomes increasingly longer, wider 15 Claims. (01. 22-1135) and deeper until finally the mold is no longer considered fit for further use. When the interior mold surface has become so fissured that it can no longer be used, it must either be replaced outright or reworked for further service use.

Prior to the present invention, various proposals as typified by the disclosure in United States Patents Nos. 1,485,859 and 1,746,373 have been made to overcome these drawbacks but within our knowledge none have proven commercially successful.

An object of the present invention is to provide an improved pipe mold having a longer useful life with the obvious attendant advantage of a reduction in mold costs of the pipe manufacturer as well as a novel method of improving the interior surface of the mold.

Another object is to provide a pipe mold that exhibits a high resistance to formation of fiscomes in contact with the rotating mold surface.

This is of great importance. With relatively smooth surfaced molds, there is often slippage between the molten metal and mold surface due to the rotational forces involved causing undesirable laps or -runs to appear on the surface of the molded pipe.

Another important advantage and which will be later discussed at greater length is that cold working with an embossed die-wheel pre-stresses the mold and better enables it to withstand the cyclic casting stresses to which it is subjected thus retarding the formation of fissures. Moreover any fissures that may ultimately occur and which are most likely to form at or near the bottom of an indentation will tend to be stopped by the land separating that particular indentation from its neighbors. Or, if the fissure should extend over into a neighboring indentation, it is most probable that its axis will follow a relatively non-harmful zig-zag courserather than a straight one which latter is by all means to be avoided because it serves as a stress-raiser and thereby favors extension of the crack along the same axis which ultimately terminates the use: fulness of the mold.

Another advantage inherent in the improved mold is thatindentations form a better gripping surface for the coating applied to the mold. Furthermore, the change in curvature of the mold surface at the lands separating adjacent indentations, tends to limit any flaking off of the coating to the immediate area where it first occurs which is of advantage since a small bare spot here or there on the mold wall can well be tolerated and will cause no noticeable defect in the exterior surface pattern of the pipe which takes on an overall and most attractive finish complementary to the particular indentation pattern worked into the mold surface.

Still another advantage inherent in the improved mold is that the indentations serve also as pockets within which gases may be entrapped and this helps to reduce transfer of heat from the molten pipe metal to the mold wall.

In the accompanying drawings which illustrate preferred mold constructions embodying the present invention, Fig. 1 is a longitudinal view in elevation of the pipe mold with part of the mold wall broken away to show a portion of the cold worked interior surface of the mold;

Fig. 2 shows a fragmentary portion of the interior of the mold illustrated in Fig. 1 magnified ten diameters to more clearly show the indiscrete character of the cold worked surface indentations;

Fig. 3 is a representative section through the mold wall of Fig. 2;

Fig. 4 is a view similar to Fig. 2 illustrating a modified form of the invention wherein the indentations are discrete.

Fig. 5 is a section taken on line 5-5 of Fig. 4;

Fig. 6 is a view similar to Fig. 2 illustrating another pattern of discrete indentations; and

Fig. 7 is an end elevation of the die head used for working the indentations into the face of the mold.

Referring now to the drawings and in particular to Figs. 1-3, the pipe mold designated by numeral l is seen to be a cylindrical tube which is relatively long as compared with its diameter and one end may be enlarged at 2 so as to provide a bell mouth on the pipe cast by the mold. The interior surface of the mold, part of which is shown in Figs. 2 and 3 is cold worked to a finish characterized by indiscretely arranged and irregularly contoured indentations 3. As seen in Fig. 3, the root sections 3a of the indentations are relatively flat and the depth may be varied somewhat as desired. If a rather bold pebble finish on the pipes cast in the mold is desired, the indentations will be worked correspondingly deep, and if a less pronounced pebbling is desired, the indentations will of course be made more shallow. However, the depth of the depressions must never be so great as to create a problem in separating the pipe from the mold which is accomplished by holding one end of the pipe stationary and moving the mold by running back the carriage on which it is supported to the starting position.

While the indentation pattern may be cold worked into the face of the mold by various methods, such as by use of an automatic peaning hammer, the preferred method, and which is performed by the apparatus disclosed in the copending application of Herbert L. MacBride, Ser. No. 106,077 filed July 21, 1949, is to use a rotatable head 4 as shown in Fig. '7 carrying a plurality of embossed die wheels 5 that are forced radially outward in the head and roll in surface contact with the mold under uniform pressure.

To start the indentation process. the die head 4 4 is set at one end of the mold with the die wheel axis arranged parallel with the longitudinal axis of the mold. The mold is then set into rotation and simultaneously the die head is given a very slight axial lead so that as the die wheels follow each other in pressure contact with the mold surface, the indentation pattern will be worked into the mold surface in a progressive manner from one end to the other. The advantage of using a plurality of dies is that it reduces the die wheel pressure required to work the surface indentations to the desired depth to the order of 1/11 where n is the number of die wheels.

Due to the intensive cold working of the interior mold surface the crystals of the mold metal become-tightly packed to an appreciable depth and are compressed beyond their elastic limit. The radial pressure applied by the die wheels, however, places the crystals in the deeper layers of the mold wall, i. e. those layers closer to the outer surface of the mold, in tension within their elastic limit. The restoring stresses in these layers react upon and tend to hold the compressed layers near the inner mold surface in compression. When the mold wall is heated by the molten pipe metal, the inner compressed layers tend to relieve their stresses in compression but since such stress relief is opposed by the tension stresses in the outer layers, .and the inner layers must first be entirely relieved of compression stresses before they can begin to take on stresses in tension which ultimately lead to the formation of fissures it is evident that the cold working will be most helpful in delaying the appearance of fissures in the interior surface of the mold.

In lieu of the irregular pattern of indiscretely arranged indentations as shown in Figs. 1, 2 and 3, a regular pattern consisting of discrete and closely adjacent, polygonally configured, indentations may be similarly cold worked into the mold surface. Such a pattern is shown in Figs. 4 and 5 from which it will be observed that each of the indentations 6 is discrete i. e. mutually isolated from its neighbors by lands I. Viewed in plan, the indentations 6 are seen to be rectangular with all four sides of equal length and two sides arranged parallel to the mold axis. All four walls of the indentations slope inwardly so as to establish a flat rectangular root portion 6a, the width of the root portion being substantially equal to the width of the lands 1 as measured across the top of the indentations. If desired, the top and bottom edges of the indentations may be rounded off slightly as indicated at lib to facilitate separation of the pipe from the mold.

As in the case of the irregular type of pattern shown in Figs. 1-3, the dimensions are not too critical and may be varied somewhat subject, however, to the limitations previously mentioned. Rectangles measuring .06" on a side at the top, and from .005" to .010" deep have proven satisfactory.

The rectangular pattern can be satisfactorily cold worked into the mold surface by a plurality of die wheels using the same technique as that heretofore described for impressing the irregular pattern, the only difference being that the periphery of each die wheel is provided with one or more rows of regularly arranged bosses, as distinguished from the irregular embossed surfaces used when making the irregular indentation pattern first described. Care must be exercised to make certain that each die wheel is accurately aumas set so that by the time the die head has made one complete revolution around the inner surface of the mold so as to leave one row or rowsof symmetrical indentations, the die head will have advanced a suificient distance axially to start the next row' or rows in the series with the lands separating adjacent rows of the same width as the lands separating adjacent indentations of the same row. Thus when the pattern has been completed the rows of indentations as viewed circumferentially or axially extend Y spirally throughout the entire surface of the mold.

Another type of discrete indentation pattern is illustrated in Fig. 6. Here the indentations 8 have a triangular configuration with the three side walls 80 sloped to terminate at a common root point 8b.. Adjacent indentations in adjacent rows lie parallel and substantially perpendicular to the mold axis. Such an arrangement which is also the case of the embodiment illustrated in Figs. 4 and 5 is preferred since it affords the most simple way of maintaining a close but accurate spacing between the indentations in adjacent rows.

In conclusion, it is to be understood that the above described indentation patterns are typical only and hence do not by any means constitute all of the possible pattern variations which can be used. Also as regards the discrete type of pattern while polygonal configurations seem most practical, those having arcuate peripheries such as circular and elliptical can be adopted.

Having now fully described our invention, what we claim is:

1. In the manufacture of metal pipe molds for use in the art of centrifugal casting, the method of finishing the interior molding surface thereof which comprises the step of cold working said surface with embossed die means rolling in pressure contact therewith to harden the mold metal to a substantial depth below said surface and simultaneously establish substantially contiguous indentations in said surface, the depth of said indentations being from about .005" to about .010".

2. In the manufacture of metal pipe molds for use in the art of centrifugal casting, the method of finishing the interior molding surface thereof which comprises the step of cold working said surface with embossed die means rolling in pressure contact therewith to harden the mold metal to a substantial depth below said surface and simultaneously establish in said surface an indiscrete arrangement of substantially contiguous indentations, the depth of said indentations being fron i about .005" to about .010".

3. In the manufacture of metal pipe molds for use in the art of centrifugal casting, the method of finishing the interior molding surface thereof which comprises the step of cold working said surface with embossed die means rolling in pressure contact therewith to harden the hold metal to a substantial depth below said surface and simultaneously establish in said surface a pattern of closely adjacent and mutually isolated indentations, the depth of said indentations being from about .005" to about .010".

4. In the manufacture of metal pipe molds for use in the art of centrifugal casting, the method of finishing the interior molding surface thereof which comprises the step of cold working unit areas of said surface with each of a plurality of embossed dies rolling successively in pressure contact therewith to. harden the mold metal to a.

6 substantial depth below said surface and simultaneously establish in said surface substantially contiguous indiscretely arranged indentations, the depth of said indentations being from about .005" to about .010".

5. In the manufacture of metal pipe molds for use in the art of centrifugal casting, the method of finishing the interior molding surface thereof which comprises the step of cold working unit' areas of said surface with each of a plurality of embossed dies rolling successively in pressure contact therewith and which track one another to harden the mold metal to a substantial depth below said surface and simultaneously establish in the surface a pattern of closely adjacent and mutually isolated indentations, the depth of said indentations being from about .005" to about .010".

6. A metal pipe mold for use in the art of centrifugal casting having cold worked indentations in its interior molding surface, the depth of said indentations being from about .005" to about .010".

7. A metal pipe mold for use in the art of centrifugal casting having cold worked and indiscretely arranged indentations in its interior molding surface, the depth of said indentations being from about .005". to about .010".

configuration.

8. A metal pipe mold, for use in the art of centrifugal casting having cold worked indentations from about .005" to about .010" deep in its interior molding surface, said --.indentations being closely adjacent and mutually isolated from one another.

9. A metal pipe mold', for use in theart of centrifugal casting havin"g .cold worked indentations from about .005" to about .010" deep in its interior molding surface, said indentations being closely spaced, mutually isolated and of polygonal configuration.

10. A metal pipe mold as defined in claim 9 wherein the sides of-adjacent indentations lie parallel.

11. A metal pipe mold as defined in claim 9 wherein the sides of indentations in adjacent rows lie parallel.

12. A metal pipe mold as defined in claim 9 wherein said indentations are of quadrilateral 15. A metal pipe mold as defined in claim 9 wherein said indentations are of rectangular configuration. 1

14. A metal pipe mold as defined in claim 9 wherein said indentations are of triangular configuration.

13. A metal pipe mold as defined in claim 9 wherein said indentations have arcuate peripheries such as circular or elliptical configurations.

RALPH E. LUDWIG. HERBERT 1.. MAcBRIDE.

REFERENCES crran The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 409,049 Lewis Aug. 13, 1889 1,006,600 Speller Oct. 24, 1911 1,707,400 Kerruish Apr. 2, 1929 1,729,747 Palm Oct. 1, 1929 1,746,373 Uhrig Feb. 11, 1930 1,833,025 Langenberg Nov. 24, 1931 2,018,762 Kauflman Oct. 29, 1935 Certificate of Correction Patent No. 2,577,423 December 4, 1951 RALPH E. LUDWIG ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 6, line 55, for the claim reference numeral 9 read 8; line 49, same column, for the claim number 15 read 13; line 55, for 13 read 15 and that the said Letters Patent should be read as corrected above, .so that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 4th day of March, A. D. 1952.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

